Method of bending or straightening a continuously cast metal strand with controlled cooling

ABSTRACT

A method is disclosed for deforming, as by bending or straightening, a continuously cast metal strand and preventing internal cracking of such a strand produced in a machine having a primary cooling zone within an open-ended mold and a curved secondary cooling zone. The bending or straightening is applied to the strand at a location where the strand has a still liquid core defining interfaces between the liquid core and surrounding solidified metal. The bending or straightening elongates and places in tension one strand wall while an oppositely facing strand wall is placed in compression. A neutral force axis, representing a location of neither elongation nor compression, is located between the wall in tension and the wall in compression. Water spray secondary cooling is applied to the oppositely facing walls at the bending or straightening location with this cooling being controlled to cool the wall in compression to a lesser degree than wall in tension, to relatively weaken the wall in compression and strengthen the wall in tension. This controlled cooling shifts the neutral axis toward the elongated tensioned wall to thereby reduce elongation of the adjacent internal interface surface and eliminate crack formation along the adjacent internal interface surface. Pinch rolls may also be arranged to apply compression to additionally shift the neutral axis toward the elongated tensioned wall.

United States Paten [191 Schiiffmann Nov. 11, 1975 METHOD OF BENDING 0R STRAIGHTENING A CONTINUOUSLY CAST METAL STRAND WITH CONTROLLED COOLING [7:5] lnventor: Rudolf Schoffmann, Linz, Austria [73] Assignee: Allis-Chalmers Corporation,

Milwaukee, Wis.

[22] Filed: Jan. 2, 1974,.

[21] App]. No.: 430,193

[30] Foreign Application l riority Data Mar. 15, 1973 Austria .J 2279/73 [52] US. Cl. 16 /89; 164/282; 164/283 S [51] Int. CL P221) 11/12 [58] Field Of Search 164/82, 89, 282, 283 R, 164/283 S 56] References Cited UNITED STATES PATENTS 3,339,623 9/1967 Rys et al. 164/89 X 3,566,951 3/1971 Schrewe 164/82 3.656.536 4/1972 Colombo 164/89 3,707,180 12/1972 Vogt 164/82 Prinmry Examiner-R. Spencer Annear Attorney, Agent, or FirmArthur M. Streich [57] ABSTRACT A method is disclosed for deforming, as by bending Or straightening, a continuously cast metal strand and preventing internal cracking of such a strand produced in a machine having a primary cooling zone within an Open-ended mold and a curved secondary cooling zone. The bending or straightening is applied to the strand at a location where the strand has a still liquid core defining interfaces between the liquid core and surrounding solidified metal. The bending or straightening elongates and places in tension one strand wall while an oppositely facing strand wall is placed in compression. A neutral force axis, representing a location of neither elongation nor compression, is located between the wall in tension and the wall in compression. Water spray secondary cooling is applied to the oppositely facing walls at the bending or straightening location Wlthfthls cooling being controlled to cool the wall in compression to a lesser degree than wall in tension, to relatively weaken the wall in compression and strengthen the wall in tension. This controlled cooling shifts the neutral axis toward the elongated tensioned wall to thereby reduce elongation of the adjacent internal interface surface and eliminate crack formation along the adjacent internal interface surface. Pinch rolls may also be arranged to apply compression to additionally shift the neutral axis toward the elongated tensioned wall.

5 Claims, 5 Drawing Figures METHOD OF BENDING OR STRAIGHTENING A CONTINUOUSLY CAST METAL STRAND WITH CONTROLLED COOLING CROSS REFERENCE TO MY COPENDING PATENT APPLICATION A method for bending or straightening a continuously cast strand of metal to eliminate internal cracks, by applying compression forces as disclosed in this ap plication, but without the controlled secondary cooling of the present invention, is the subject of my copending patent application entitled Method of Bending or Straightening a Continuously Cast Metal Strand Having a Still Liquid Core", Ser. No. 430,040 filed concurrently with this application.

BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a method for bending and/or straightening continuously cast slabs, billets and related products which have a basically quadrangular, normally rectangular, cross section. The method of operation of a continuous casting machine to which the present invention may be applied, involves charging a casting mold with liquid metaland precooling the metal in the mold for the purpose of forming a strand having a closed shell of solidified metal surrounding a still liquid core, with the strand leaving the mold and being bent and/or straightened after it has left the mold.

2. Description of the Prior Art It is known to those familiar with this technology, and disclosed in my prior patents US. Pat. No. 3,589,429 of June 29, 1971 and US. Pat. No. 3,710,847 of Jan. 16, 1973, that a strand with only a relatively thin shell of solidified metal is formed in the mold of continuous casting plants. Complete solidification of the strand occurs in the so-called secondary cooling zone after leaving the mold. In order that a casting machine may be built of minimum vertical height, the secondary cooling takes place with the strand moving downwardly along a curved bow which delivers the strand along a horizontal path. If the mold defines a straight and vertical cavity the strand emerging from the mold is first bent and later straightened. If the mold defines a cavity which forms the strand with the curvature of the bow, the strand is later straightened. In either case the strand may be deformed, as by bending and/or straightening,

while the strand has a still liquid core. In continuous casting plants built and operated according to methods taught by the prior art, the bending and straightening of strands often caused internal cracks or internal defects resulting from welded internal cracks. The latter internal defects are sometimes referred to in prior art literature as ghostlines.

SUMMARY OF THE PRESENT INVENTION The object of the present invention is the prevention of internal cracks or other internal defects caused by cracks.

The present invention is based upon the discovery that the cause of internal cracks and internal defects caused by cracks, is the extremely small allowable elonv A method according to the present invention involves deeforming, as by bending or straightening, a continuously cast metal strand and preventing internal cracking of such a strand produced ina machine having a primary cooling zone within an open-ended mold and a curved secondary cooling zone. The bending or straightening is applied to the strand at a location where the strand has a still liquid core defining interfaces between the liquid core and surrounding solidified metal. The bending or straightening elongates and places in tension one strand wall while an oppositely facing strand wall is placed in compression. A neutral force axis, representing a location of neither elongation nor compression, is located between the wall in tension and the wall in compression. Water spray secondary cooling is applied to the oppositely facing walls at the bending or straightening location with this cooling being controlled to cool the wall in compression to a lesser degree than wall in tension, to relatively weaken the wall in compression and strengthen the wall in tension. This controlled cooling shifts the neutral axis toward the elongated tensioned wall to thereby reduce elongation of the adjacent internal interface surface and eliminate crack formation along the adjacent internal interface surface. Pinch rolls may also be arranged to apply compression to additionally shift the neutral axis toward the elongated tensioned wall. The compression applied by the pinch rolls, of course, also increases the compression of the wall already in compression but such an increase in compression does not cause cracks.

Thus the interface surface which is in the aforesaid critical temperature range that allows little elongation, and which is nearest to the strand wall in tension, is itself subjected to little or no elongation and cracking along this critical interface is eliminated.

Other features and objects of the invention that have been attained will appear from the more detailed description to follow with reference to an embodiment of the present invention shown in the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 of the drawing is a typical curve showing the allowable elongation of a steel depending upon temperature;

FIG. 2 shows schematically a fragmentary cross section view of a strand forming a slab, with apparatus operable according to the present invention;

FIG. 3 shows an elongation-compression pattern in a ,cast strand as theoretically occurs when deforming the strand, as by bending, according to a method taught by the prior art;

FIG. 4 shows an elongation-compression pattern in a cast strand as actually occurs when deforming the strand, as by bending, according to a method taught by the prior art and accounting for superimposed strand withdrawal forces; and

FIG. 5 shows an elongation-compression pattern in a cast strand when deforming the strand, as by bending, according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT As hereinbefore stated, the object of the present invention is the prevention of internal cracks or other internal defects caused by cracks, and the present invention is based upon the discovery that the cause of internal cracks and internal defects, is the extremely small allowable elongation in a critical temperature range near the solidus temperature of the metal. The typical curve of the allowable elongation of a steel depending on temperature is shown in FIG. 1 of the drawing. According to this diagram, the allowable elongation in the critical temperature range K near the solidus temperature ('s) is only a small fraction of the elongation for the balance of the temperature range. In this critical temperature range, the absolute values of the allowable elongation vary for different grades of steel, but will reach values of only 0.l percent for some grades.

FIG. 2 shows schematically a cross section of a part of a cast strand during bending.

The cast strand 1 shown in FIG. 2 emerges from an open-ended water-cooled casting mold 2. The strand 1 emerges vertically downward and is then bent in the secondary cooling zone by support rolls 3, where such as water sprays 4 continue cooling strand 1. The cast strand has a liquid core which becomes smaller with increasing distance from the casting mold 2 and an already solidified shell 6, with a shell thickness S and zones K which increases correspondingly, and with a predetermined strand thickness D. The zones K, which are in the critical temperature range, develop at the solidification fronts F and F which are the interface surfaces between the liquid core 5 and the solidified shell 6. The neutral bending axis of the strand is designated n, the surface temperature '0 and the solidus temperature s.

The cast strand 1 has no curvature when entering the bending zone, the strand curvature R is therefore whereas the curvature of strand 1 corresponds to the bending radius R when it exits the bending zone. In the bending zone the elongation of the critical zone K must not exceed the maximum allowable values arising from FIG. 1 if interior cracks or interior defects are to be avoided. It may be assumed that the cast strand enters the critical zone at A and leaves the critical temperature bending zone at B. The bending radius at A shall be "A and "B at the point B. In conventional procedure, a strand element is elongated on its way from A to B on the tension side of the bending zone. This elongation can be calculated according to the following formula.

According to this formula, necessary small elongations can only be obtained if the radii "A and "B are very large, the strand thickness D is very small and the change in radii from "A to "B is also very small.

Large radii and small changes in radii result in very high structures for continuous casting machines. Increasing height in continuous casting machines results in increasing fe'rrostatic pressure which applies excessive stress to the strand and the machine. The strand thickness D is usually determined by other factors which cannot be changed arbitrarily.

The elongation-compression patterns in the cast strand during bending are shown in FIGS. 3, 4 and 5, with the compression side on the right and the tension side on the left hand side of the drawing. In FIG. 3 the uninterrupted diagonal XX which crosses the centerline NN shows that the theoretical distribution of the elongation and compression according to prior art knowledge is symmetrical around the centerline NN with the elongation, between diagonal XX and a line YY perpendicular to NN, in the area of the critical zone K already reaching a considerable value. Actual conditions in continuous casting plants according to prior art practices, as shown in FIG. 4 are worse since withdrawal forces, which act on the strand, are superimposed as a tension stress on the bending stresses. As shown in FIG. 4, this moves the neutral axis into the position N -N toward the compression side and correspondingly causes the tension stress and elongation in the critical area K to be larger, and it is such elongation at the interface surface F in FIG. 4, that causes cracks.

The manner in which the present invention solves the aforesaid problem will be described with reference to FIGS. 2 and 5. Referring first to FIG. 2, the deformation (bending in this case) of strand 1 by the support rolls 3 elongates and places strand wall 7 in tension and the oppositely facing strand wall 8 is placed in compression while water sprays 4 apply secondary cooling. A pair of pinch rolls l0 and 11 may be arranged ahead of the bending zone. Roll 10 engages the elongated and tensioned wall 7 and roll 11 engages the wall 8 which is in compression. A second pair of pinch rolls l2 and 13 may be arranged beyond the deformed zone. Roll 12 engages wall 7 and roll 13 engages wall 8. Each of the rolls 10, ll, 12 and 13 is provided with a suitable means for applying a force toward the strand 1. Such means may be, for example, fluid pressure operated devices 14, 15, 16 and 17, each operable to move a roll connected thereto as indicated by arrows.

In the operation of such an apparatus as shown in FIG. 2, according to the method of the present invention, the water sprays 4 cool the oppositely facing strand walls 7 and 8, and this cooling is controlled to establish the surface temperature of the wall 8 which is in compression, at a level of from about 40, to 400F, higher than the temperature of wall 7 which is in tension. Another operation is to cool both strand surfaces differently before they enter the deformation zone such that the wall thickness differs by more than 5 percent (thinner on the compression side). In this case the aforesaid problem is solved too, even if temperatures in the deformation zone are equal on both sides. In other words the wall 8 in compression is therefore weaker than the wall 7 in tension and the differential in wall thickness is more than about 5 percent. This cooling may be so controlled by providing a greater number of sprays 4 to cool wall 7 than is provided to cool wall 8 as shown in FIG. 2 and/or by regulating the flow of water through sprays 4. The wall 8 in compression is therefore weaker than wall 7 because it is thinner and/or hotter. weakening wall 8 does not cause cracks because wall 8 is in compression. The relative weakening of the wall 8 in compression and strengthening of the wall 7 has the effect of moving the diagonal line XX to the left as shown in FIG. 5, i.e. toward the wall 7 in tension, and moves the neutral axis to the position indicated by line FI -N in FIG. 5. The neutral axis N N in FIG. 5 can thusly be moved close to or even coincident with interface surface F adjacent wall 7 and thereby reduce elongation along the interface F to substantially zero. Additionally, rolls 10, l1, l2 and 13 may be moved by the devices l4, l5, l6 and 17 to each apply a force toward strand 1 to apply compression to strand 1 which acts to reduce the tension on wall 7 and increase compression of wall 8. This also has the effect ofmoving diagonal line XX to the left, i.e. toward wall 7 in tension, and also results in moving the neutral axis N N closer to the interface surface F to reduce elongation along interface surface F The compression applied by the pinch rolls 10, 11, 12 and 13 also increases the compression of the wall 8 which is already in compression due to bending, but such increase in compression does not cause cracks.

"Thus, the interface surface F which is in the aforesaid critical temperature range K that allows little elongation, and which is nearest to the strand wall 7 in tension, is itself subjected to little or no elongation and cracking along this critical interface is eliminated. The object of the present invention is thereby achieved.

From the foregoing detailed description of the present invention it has been shown how the object of the present invention has been attained in a preferred manner. However, modification and equivalents of the disclosed concepts such as readily occur to those skilled in the art are intended to be included in the scope of this invention. Thus, the scope of the invention is intended to'be limited only by the scope of the claims such as are or may hereafter be, appended hereto.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. A method of preventing internal cracking in a continuously cast metal strand produced in a machine having a primary cooling zone within an open-ended mold, a curved secondary cooling zone, and strand deforming means for changing the radii of a pair of oppositely facing external walls of the solidifying strand; comprising the steps of:

a. deforming the strand to change the strand radii at a location where the strand has a still liquid core defining interface surfaces between the liquid core and surrounding solidified metal, thereby elongating and placing one of said oppositely facing external walls in tension and placing the other of said oppositely facing external walls in compression and 6 with a neutral axis between said wall in tension and said wall in compression;

b. preconditioning the strand prior to deforming by applying an external compression force to the wall in tension to shift the neutral axis toward the elongated wall; and

c. applying controlled secondary cooling to said oppositely facing walls to establish the temperature of said wall in compression at a level of 40 to 400F higher than the temperature of said wall in tension and to relatively weaken the wall in compression and strengthen the wall in tension, and continue the progressive shifting of the neutral axis toward the elongated wall which began with the application of compression force according to step (b), and thereby reduce elongation of the adjacent internal interface surface and reduce crack formation along said adjacent internal interface surface.

2. A method according to claim 1 in which the strand deforming means are arranged in close spaced relation to the primary cooling zone to receive a straight strand and deform the strand by bending the strand to a curvature for passage through the secondary cooling zone, and the secondary cooling is applied by spraying water on the strand walls and at a greater rate on the wall in tension than is sprayed upon the wall in compression.

3. A method according to claim 1 in which said external compression force is applied by arranging a pair of pinch rolls with one roll engaging the elongated tensioned strand wall and the other roll engaging the strand wall in compression, and applying forces to said rolls tending to move said rolls toward each other.

4. A method according to claim 3 in which said rolls are arranged to engage the strand walls before the continuously moving strand arrives at the deforming location.

5. A method according to claim 4 in which a second pair of pinch rolls are arranged and forces are applied thereto in the manner of the first pair but with the second pair being spaced from the first pair in the direction of strand movement. 

1. A method of preventing internal cracking in a continuously cast metal strand produced in a machine having a primary cooling zone within an open-ended mold, a curved secondary cooling zone, and strand deforming means for changing the radii of a pair of oppositely facing external walls of the solidifying strand; comprising the steps of: a. deforming the strand to change the strand radii at a location where the strand has a still liquid core defining interface surfaces between the liquid core and surrounding solidified metal, thereby elongating and placing one of said oppositely facing external walls in tension and placing the other of said oppositely facing external walls in compression and with a neutral axis between said wall in tension and said wall in compression; b. preconditioning the strand prior to deforming by applying an external compression force to the wall in tension to shift the neutral axis toward the elongated wall; and c. applying controlled secondary cooling to said oppositely facing walls to establish the temperature of said wall in compression at a level of 40* to 400*F higher than the temperature of said wall in tension and to relatively weaken the wall in compression and strengthen the wall in tension, and continue the progressive shifting of the neutral axis toward the elongated wall which began with the application of compression force according to step (b), and thereby reduce elongation of the adjacent internal interface surface and reduce crack formation along said adjacent internal interface surface.
 2. A method according to claim 1 in which the strand deforming means are arranged in close spaced relation to the primary cooling zone to receive a straight strand and deform the strand by bending the strand to a curvature for passage through the secondary cooling zone, and the secondary cooling is applied by spraying water on the strand walls and at a greater rate on the wall in tension than is sprayed upon the wall in compression.
 3. A method according to claim 1 in which said external compression force is applied by arranging a pair of pinch rolls with one roll engaging the elongated tensioned strand wall and the other roll engaging the strand wall in compression, and applying forces to said rolls tending to move said rolls toward each other.
 4. A method according to claim 3 in which said rolls are arranged to engage the strand walls before the continuously moving strand arrives at the deforming location.
 5. A method accordinG to claim 4 in which a second pair of pinch rolls are arranged and forces are applied thereto in the manner of the first pair but with the second pair being spaced from the first pair in the direction of strand movement. 